A Workflow can be setup to trigger on either a user or an order. However, when setting up specific Workflow routes, please note that custom fields can only be selected if the specific Workflow triggers on users, not orders. In the below example - the 'Spells' Workflow is triggered on users. You are able to configure a Workflow route on the custom field 'Spell cast':

However, if the Workflow is triggered on Orders - you are currently unable to add a route using a custom field: